How do you download mp3 ttracks?

My wife has just bought a Samsung J700 phone and after an hour long tussle to remove the back plate I thought I might forestall other problems now. The manual has to be the worst I've seen, only basic instructions but beyond that zilch so any advice on downloading music and what is the biggest micro sd card it will take? The main reason for her buying this was to use it as an mp3 player as well as a phone as it seemed cheaper than an ipod- but now I am not too sure. Any help would be appreciated.